Add isNoopScheduler with import-order guard in test harness

Author: ofriw

runtime-singletons.ts

@@ -91,3 +91,8 @@ export function __setSingletons(
 export function getSingletons(): RuntimeSingletons {
 	return current;
 }
+
+/** True when scheduler is the pre-init noop — see createTestHarness() safety check. */
+export function isNoopScheduler(scheduler: RuntimeFrameScheduler): boolean {
+	return NOOP_SCHEDULER_MARKER in scheduler;
+}

tests/test-utils.ts

@@ -26,6 +26,7 @@ import {
 	__setSingletons,
 	createWriteLock,
 	getSingletons,
+	isNoopScheduler,
 	type RuntimeSingletons,
 } from "../runtime-singletons.js";
 import { SpawnFrameScheduler } from "../spawn/renderer.js";
@@ -68,6 +69,17 @@ export function createTestHarness(): TestHarness {
 	const originalWarn = console.warn;
 	const originalError = console.error;
 
+	// Check whether spawn/renderer.ts was already statically imported before
+	// this harness call — if previousSingletons still holds the noop marker,
+	// the production registration at the bottom of spawn/renderer.ts never ran.
+	if (isNoopScheduler(previousSingletons.frameScheduler)) {
+		console.warn(
+			"[test-utils] spawn/renderer.ts was not statically imported before " +
+				"createTestHarness() — the production frame scheduler was never " +
+				"registered. Frame-batched rendering tests will use the noop scheduler.",
+		);
+	}
+
 	// Atomic swap: replace the production singleton container (write lock,
 	// context, frame scheduler) in one call.
 	__setSingletons(singletons);

tests/unit/runtime-singletons.test.ts

@@ -6,6 +6,7 @@ import {
 	__setSingletons,
 	createWriteLock,
 	getSingletons,
+	isNoopScheduler,
 } from "../../runtime-singletons.js";
 
 test("createTestHarness swaps singleton state atomically and restores it on teardown", () => {
@@ -83,3 +84,15 @@ test("write lock serializes concurrent writers and completes all", async () => {
 
 	h.teardown();
 });
+
+test("isNoopScheduler returns false for SpawnFrameScheduler", () => {
+	// The noop scheduler is created at module init but overwritten by
+	// spawn/renderer.ts at import time — so the global singleton always
+	// holds a real scheduler. The true path (returns true) is exercised by
+	// the import-order guard in createTestHarness() — see test-utils.ts.
+	assert.equal(isNoopScheduler(getSingletons().frameScheduler), false);
+
+	const h = createTestHarness();
+	assert.equal(isNoopScheduler(getSingletons().frameScheduler), false);
+	h.teardown();
+});